* fix(s3): stamp the gateway's own ids on single-shot PutObject entries
putToFiler builds the entry in the gateway now instead of proxying a PUT to
the filer, and it hardcoded Uid/Gid 0 while every sibling write path stamps
filer_pb.OS_UID/OS_GID. On a non-root deployment that leaves single-shot PUTs
and multipart parts owned by root while directories and completed multipart
objects keep the real ids, so a mount reader can list the tree but gets EACCES
on every open once objects are not world-readable.
* fix(s3): stamp mode and ownership on copy destinations
CopyObject and UploadPartCopy build the destination attributes themselves and
then assign them over the entry filer_pb.MkFile just stamped, so the copy
landed with mode 0000 and uid/gid 0 - unreadable on a mount even by the filer's
own user. Build the destination with the same mode PutObject resolves for the
request and the gateway's own ids.